If you look at the image below you will see that the very edge of my taskbar is showing at the bottom of the screen. This also sometimes appears in full screen mode when watching videos. I tried resetting windows explorer and resetting my taskbar settings but that didn't seem to work. You might need to open image in new tab to if you can't see what I'm talking about in the image.
This behavior is normal and by design (at least part of it). Let me explain....
The taskbar is a visual element. In order for it expand the top of the mouse pointer has to be able to make contact with the taskbar. If not for the couple of pixels still visible when hidden, your mouse could never make contact with that display element and it could then not cause any action (in this case, unhiding it).
In my case, if I am playing a video, say for example with VLC, and the player is in full screen mode, then my taskbar is completely 100% hidden. In that state, if I move my mouse pointer all the way to the bottom of the screen, the taskbar never appears because the mouse pointer has no taskbar to point to.
The fact that you are still seeing the edge of the taskbar when playing a video full screen leads to believe that in all liklihood there is
1) A problem with the video app.
2) a problem with the display driver.
Try maybe using another video player to see if they all have this problem. If so, then move on to looking at the video driver. In other words, it may take a little process of elimination here.
With all that said, just to emphasize, in normal operation, you WANT a little peek (a couple pixels maybe) of the taskbar so that there is something for the pointer to interact with.
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